Increasing the rolling contact fatigue life is essential to achieving high reliability of bearings. It is well known that the life of bearings is significantly affected by non-metallic inclusions. Rolling contact fatigue life is classified into two types depending on the required reliability. One is known as the L10 life that serves as a common measure of bearing performance, and the other is known as the short life that is determined based on accidental failures. Specifically, the term “short life” is defined as the length of bearing life at which a bearing fails prematurely, or earlier than its calculated life span. Since the L10 life and the short life are determined based on two different types of failures that occur at significantly different frequencies, the difference in the frequency of these failures is thought to be due to inclusions occurring at significantly different frequencies. Accordingly, a proper inspection volume for inclusion evaluation (reliability evaluation) must be selected depending upon the type of inclusions, or the type of bearing life.
The L10 life of bearings is considered to be determined by small- or medium-sized inclusions, specifically inclusions of approximately 100 μm or less in size (primarily several to several tens of microns). The short life of bearings is considered to be determined by large inclusions sized approximately 100 μm or greater. Thus, a system needs to be constructed that can evaluate the two types of inclusions and can thus be used to define high-reliability steel.
Techniques conventionally used to evaluate the cleanliness of steel are microscopy designed for the evaluation of non-metallic inclusions in steel as specified in JIS G0555, microscopy designed for the direct observation of polished specimens using a microscope as specified in ASTM E45, and the acid-solution technique, in which diluted HNO3 or the like is used to dissolve iron matrix to extract the inclusions from steel for observation.
One technique commonly used to detect inclusions present in steel is the ultrasonic flaw detection technique. Patent Document 1 describes a rolling element of a power roller bearing in which the maximum size of the inclusions present in the element at twice the depth of the maximum shear stress position or lesser depths is limited to 200 μm or less.
Patent Document 2 describes a high-cleanliness steel in which the number of 20 μm or larger oxide inclusions detected by acid-dissolving of the steel is 40 or less per 100 g of the steel.
Patent Document 3 describes steel with stable heat treatment distortion in which it is ensured that the number of inclusions that have a √AREA of greater than 100 μm is 2 or less per 1.0×105 mm3 of the steel.
Patent Document 4 describes a technique in which test specimens are prepared by rolling and/or forging a steel at a rolling and/or forging ratio of 6 or higher and normalizing or annealing the steel and are then subjected to water-immersed ultrasonic flaw detection using a point focused-type probe at a frequency of 5 to 25 MHz to detect the inclusions in the steel. The technique achieves improved detection accuracy.
[Patent Document 1]Japanese Patent Laid-OpenPublication No. 2004-144289[Patent Document 2]Japanese Patent Laid-OpenPublication No. 2001-342512[Patent Document 3]Japanese Patent Laid-OpenPublication No. 2003-247046[Patent Document 4]Japanese Patent Laid-OpenPublication No. 2004-93227